Only replace original render callback if not set

[menno-ll]

Issue link

https://app.asana.com/0/1206316631984075/1207645749086440/f

Description

parse_blocks can crash beacuse of an infinite loop.
Somehow for the image block the constructor can be called twice, resulting in the original render callback beeing the same as the render callback.
Because they became the same, the following call to fallback to the original render callback if no custom callback is available, will then cause an infinite loop (because render callback is calling the same render callback,etc).

This solution aims to solve this by only allowing the original render callback to be set once.
It's the original, so that should always be allright.

How Has This Been Tested?

In a project, by running parse_blocks with an image block in the content present.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.